Ei Varga

Controllability and observability of heat exchanger networks in the time-varying parameter case

Abstract In this paper the structural controllability and observability of heat exchanger networks are determined from qualitative information about the heat exchangers and their network topology. The heat exchanger network is modelled as a time-varying linear system based on first engineering principles where the effect of the flowrate variations is described by unknown time-varying parameters, inlet temperature variations are regarded as disturbances and external heaters/coolers are considered as input variables. Necessary and sufficient conditions for structural controllability and observability of heat exchanger networks are derived, based on an extended Kalman-type rank criterion for linear systems with time-varying parameters. Determining the structural controllability and observability of heat exchanger networks needs only checks for the input and output connectability of the network in both constant and time-varying parameter cases. The results are extended to the more-practical case where bypass ratios are also used as control variables and where more than one time-varying parameter enters into the state-space matrices.

The effect of the heat exchanger network topology on the network control properties

Abstract The structural control properties of heat exchanger networks has been investigated from the viewpoint of network topology. While the structural stability is independent of topology, the structural controllability and observability strongly depend on topological features, like splitting and mixing connections. The dynamics of heat exchanger superstructure has been studied by numerical simulation and qualitative prediction: overshoot and inverse responses have been observed due to splitting and mixing connections.

Grey Dynamics of Heat Exchanger Networks

Abstract A robust modeling method of Heat Exchanger Networks (HENs) with structured parametric perturbations has been described in this paper which results in a LFT form. The LFT of the overall HEN system can be generated algorithmically from the model of the heat exchangers and from the network topology. Based on these modeling paradigms the effect of uncertainties represented by specific Δ block structures have been analysed by frequency domain n analysis in order to get an insight into the causes of potential large overshoot-type responses. The proposed method has been illustrated on the simple example of a countercurrent heat exchanger with by-pass.

Note on the effect of recycle on the dynamics of chemical process plants

Abstract The complex multivariate multiunit problem formulation of analyzing the effect of recycles on the dynamics of chemical process plants is considered in this Note. Based on the notion of exchange plants and results on conservation matrices, the effect of connections, such as recycles and external sources, such as chemical reaction, heating/cooling etc. is analysed. It has been found that exchange plants are not unstable, regardless of the connections of their operating units. If constant total flow is maintained through each unit then an increase of all kinds of connections including recycles at the expense of an external stream may move the eigenvalues of the plant towards the instability region. This effect can cause stable nonexchange plants to become unstable.

Probability density functions based micromixing model of chemical reactors

Abstract A mathematical model based on the probability density function approach is presented for describing micromixing in chemical reactors. Here, macromixing is described as the motion of the chemical species in the physical space while micromixing is interpreted as motion in the abstract composition space of the components. In the special case of stirred-tank reactors, the model closed by using the quasi-Gaussian hypothesis, is analyzed by means of the moment-generating equations. Preliminary computational results are presented and compared with experimental ones of Takao et al . (1979) concerning an infinite rate reaction.

Robust analysis of potential large overshoots in heat exchanger networks

A robust modeling method of heat exchanger networks (HENs) with structured parametric perturbations has been used in this paper which results in a model in LFT form. The LFT of the overall HEN system can be generated algorithmically from the model of the heat exchangers and from the network topology. Based on these modeling paradigms robust stability and performance tests have been applied using L/sub /spl infin// signal norms in a non-standard way to determine an upper bound for overshoots in HENs. The proposed method has been illustrated on the simple example of a countercurrent heat exchanger with bypass.

Toward an intelligent evolutionary signal processing based failure monitoring and diagnostic system for complex plants

Abstract This paper presents an approach of a signal processing based failure monitoring and diagnostic system which can be applied in complex, partially unknown, time-varying plants. In these types of systems the diagnostic knowledge built in cannot be static, it must be evolved as the plant is varying and/or the operating experience is growing. Therefore this approach concentrates on the designing - knowledge engineering - process: offers an integrated environment to the diagnostician to make experiments, to apply a series of processing methods selected from an extensive method library, to evaluate the results, to collect and use time-dependent data of critical plant parameters, as well as to assemble examinations to solve specific failure detection tasks, and design monitoring processes. Realization aspects are also outlined.

A tool for development of qualitative and approximate quantitative process models

Abstract A methodology and a tool to support the early stages of model development is presented. The model development basis is conservation of extensive thermodynamical properties, called quantities. The conceptual basis for the graphical editor layout is description of dynamic interactions between process quantities and interactions from the environment in a so called DYnamic QUantity Interaction Diagram (DYQUID). The DYQUID model representation forms a superset of a large number of dynamic process model representations, such as locally linear model structures, qualitative differential equations and conventional nonlinear state space models. The prototype version of the editor is object oriented, and is written in C++. The editor works with objects which fit engineering notions. Each object has its graphical representation. The editor is equipped with generic function types for transfer and reaction mechanisms. The editor may be expanded with additional function types suitable for a pertinent application area. The editor is interactive, driven by menus and questionnaires. One purpose of this experimental version of a DYQUID editor is to obtain experience with the efficiency of the user interface for facilitating development of different types of mutually consistent model representations.

Robust analysis of distillation column dynamics

Abstract The robust stability and performance of a binary distillation column model is analysed using a minimal state space representation. The uncertainty modelling of the column is resulted in an LFT form with a four element diagonal block structure taking into account the dependence between the steady state gains. Robust analysis of the distillation column dynamics is performed and compared using µ - and l 1 -analysis tools. The magnitude of possible overshoots is determined by l 1 -test for robust performance and the dependence of this magnitude on the dominant time constant (i.e. the purity of the separation) is investigated. Futhermore the effect of the mi certainties in the time constants and the singular values on the robust performance is also analysed using l 1 -test. It has been found that the uncertainty in the time constants affects dominantly the robust performance of the open-loop system, i.e. the magnitude of possible overshoots.

THE EFFECT OF DESIGN PARAMETERS ON ROBUST PERFORMANCE

Abstract The effect of design parameters (i.e. characteristic sizes of the equipments) of exchange plants on the robust performance has been investigated in order to use the robust performance measure for integrated process and control system design. It has been shown that for exchange plants the l 1 -norm of the impulse response can be expressed analytically. By approximating the spectral radius of the matrix ĥ used in the l 1 -analysis, an analytical formula for the effect of design parameter of heat exchanger cell, i.e. the heat transfer rate, on the robust performance measure, i.e. the magnitude of maximum possible overshoot, has been derived. It has been found that the dependence of the performance measure (the magnitude of overshoots) on the design parameter (heat transfer rate) is a rational decreasing function for one heat exchanger cell.